1. Field of the Invention
The present invention relates to a method of soldering a semiconductor substrate on a supporting plate, and particularly to an improvement for preventing deterioration in the electronic character of the semiconductor substrate due to a soldering process and for increasing the efficiency of the soldering process.
2. Background Arts
As well known in the field of semiconductor technology, a semiconductor substrate in which one or more electronic elements are formed is often soldered on a supporting metal plate in the process for fabricating semiconductor power devices or the like.
FIG. 6A illustrated an assembly 10 prepared for the process of soldering silicon substrate or wafer 2 on a supporting metal plate 4. A layered structure of an n-type silison region 2a and a p-type silicon region 2b is formed in the silicon substrate 2, whereby the silicon substrate 2 functions as a power diode having a high breakdown-voltage. The supporting metal plate 4 is provided for supporting the silicon substrate 2. The assembly 10 is constructed such that an aluminum thin plate 3 serving as a solder metal layer and the silicon substrate 2 are serially placed on the supporting plate 4 in this order and a weight 1 is further placed thereon.
FIG. 7 is a graph showing the temperature change in a furnace in which the assembly 10 is heated in order to solder the silicon substrate 2 on the supporting plate 4, and the shown temperature is substantially coincident with the temperature of the assembly 10 in the furnace. In the conventional method, the temperature in the furnace is increased from an initial temperature or a room temperature T.sub.R to 640.degree. C. in one hundred minutes (the perion P.sub.0 -P.sub.10). The temperature 640.degree. C. is higher than an eutectic reaction temperature T.sub.A =585.degree. C. being defined as a transition temperature at which the silicon and the aluminum thin plate 3 are molten to become an eutectic melt consisting of silicon and aluminum.
Then, the temperature in the furnace is maintained at 640.degree. C. for ten minutes (the period P.sub.10 -P.sub.11). Therefore, the transition to the eutectic melt starting from the interface between the silicon substrate 2 and the aluminum thin plate 3 spreads over the whole region of the aluminum thin plate 3 in the period P.sub.10 -P.sub.11, so that the aluminum thin plate 3 becomes an Al--Si eutectic melt layer. The top surface of the supporting metal plate 4 becomes wet with the Al--Si eutectic melt, whereby the silicon substrate 2 is soldered on the supporting plate 4.
Then, the assembly 10 is cooled by natural heat dissipation in the period P.sub.11 -P.sub.12 so that the Al--Si eutectic melt layer is solidified. The weight 1 is removed from the assembly 10, whereby the soldering process is completed.
FIG. 6B illustrate the silicon substrate 2 which has been soldered on the supporting plate 4 with an Al--Si eutectic solid layer 3a obtained through the above-described process.
Although the conventional technique has been widely used, the same has the following disadvantages: First, attention is directed to the flatness of the silicon substrate 2. The major surfaces of the silicon substrate 2 are shaped into mirror surfaces to a possible extent. However, it is impossible in practice to perfectly avoid irregulations and curvatures of the major surfaces. Furthermore, the surfaces of the aluminum thin plate 3 also have irregularities and curvatures. Therefore, the bottom major surface of the silicon substrate 2 in the assembly 10 is not uniformly in contact with the top surface of the aluminum thin plate 3.
Under the circumstances, the Al--Si eutectic reaction in the period P.sub.10 -P.sub.11 of the FIG. 7 starts from only the contact regions at which the silicon substrate 2 is well in contact with the aluminum thin plate 3, and then spreads to the other regions, i.e., non-contact regions. That is, the progress of the eutectic reaction is not uniform on the respective regions of the interface between the silicon substrate 2 and the aluminum thin plate 3. In accordance with the non-uniformity, the interface 5 (FIG. 6B) between the Al--Si eutectic layer 3a and the silicon substrate 2 includes flat regions 5a and projection regions 5b projecting into the silicon substrate 2, where the regions 5a and 5b correspond to the non-contact and contact regions, respectively.
As a result, the following problems are caused in the semiconductor device obtained through the soldering process.
(1) Since respective crystallographic structures of the Al--Si eutectic solid layer 3a and the silicon substrate 2 are different from each other, a non-uniform stress is caused in the portion of silicon substrate 2 close to the interface 5, which is the bottom portion of the p-type silicon region 2b in the example shown in FIG. 6B. Accordingly, the electronic character of the diode deviates from the designed character, and the mechanical strength of the diode device is decreased.
(2) When a reverse voltage is applied to the p-n junction J of the diode, the thickness of the depletion layer extending from the p-n junction J to the p-type region 2b becomes non-uniform in respective regions, since the spatial irregularities are caused in the interface 5 between the silicon substrate 2 and the Al--Si eutectic layer 3a whose respective electronic characters are different from each other. Consequently, electronic fields concentrate at the neighborhood of the projection regions 5b and the diode breaks down at a relatively low voltage.
The conventional soldering method causes not only the above-described problems but also the following other problems.
(3) Since the eutectic reaction starts from the contact regions and then spreads to the non-contact regions, the progress of the eutectic reaction is considerably slow and the supporting plate 4 cannot be sufficiently wet with the Al--Si eutectic melt unless the assembly 10 is held at a relatively high temperature in the period P.sub.10 -P.sub.11. Therefore, the maximum temperature in the furnace which appears in the period P.sub.10 -P.sub.11 must be relatively high, and accordingly, a thermal stress in the diode becomes high.
(4) In order to accelerate the spread of the eutectic reaction in the non-contact regions, the aluminum thin plate 3 should be sufficiently heated before the eutectic reaction is started. Thus, the heating speed in the period P.sub.0 -P.sub.10 should be relatively slow so that the respective regions of the aluminum thin plate 3 can be sufficiently preheated in the period P.sub.0 -P.sub.10 for the thermal activation thereof. Consequently, the total time required for the conventional soldering process is long.